Thermal stability of Solar Salt at 650 °C - A deep dive into molten nitrate salt chemistry

Abstract

At the last SolarPaces conference we demonstrated that the thermal stability of conventional Solar Salt (60-40wt % NaNO3-KNO3) is substantially increased when the storage system including the gas system is sealed.[4] The operating temperature could therefore successfully be enhanced to 600 °C. During the last months we have investigated the temperature regime above 600 °C to push the thermal stability limit even further, e.g. up to 650 °C. In closed systems all gas species relevant for regeneration of the salt (mainly O2 and NOx), will accumulate in the gas phase and shift chemical equilibria to the nitrate and nitrite side of Eq (1) and (2) (shown later), respectively. Yet, it remains unclear which ions participate in the decomposition process into oxide ions. In this study we explored the role of the nitrate and nitrite ions in the decomposition reactions at temperatures up to 650 °C to confirm their role in the overall decomposition process and the importance of gas composition.